Monolayer

monolayer

Monolayer

a layer of a substance 1 molecule thick on a surface or at a phase boundary. Monolayers are produced by adsorption, surface diffusion, and evaporation of a solvent from a solution containing a nonvolatile component. Monolayers that are formed by surface-active substances on the surface of a liquid or at the boundary between two nonmiscible liquids may exist in various two-dimensional states (gaseous, condensed, and intermediate, or “liquid-expanded”).

In gaseous monolayers, the distance between the molecules is great relative to their size; therefore, cohesive molecular interactions are virtually absent. On the other hand, condensed monolayers have limiting molecular packing density. In the case of fatty acids, alcohols, and other molecules that may be represented as hydrocarbon chains with a polar terminal group, condensed monolayers resemble “picket fences” occupying the entire surface area. Each molecule in such a “fence” is arranged perpendicularly or at an angle to the phase boundary surface and, regardless of its own length, usually occupies an area of 20–25 square angstroms. As a rule, linear macromolecular compounds form monolayers with horizontal orientation of the macromolecules. If the cohesion is sufficiently high, monolayers may have surface viscosity and strength that differ sharply from those of the bulk phases.

The structure and properties of monolayers have a strong effect on mass transfer (evaporation and diffusion), catalysis, friction, adhesion, and corrosion; this is taken into account in solving various engineering and industrial problems. The stability of highly disperse systems (sols, emulsions, and suspensions) often depends predominantly on the state of a monolayer.

Monolayers also play an important role in biological systems. For example, there are membrane structures in all cells of living organisms. Biological membranes basically consist of two monolayers of protein molecules with a double (bimolecular) layer of lipids between them. The thickness of such a four-layer membrane is 70–80 angstroms. The alternation of various types of monolayers also results in the lamellar structure of certain cell organelles, such as chloroplasts in the cells of green plants. Artificial monolayers are used as models of biological membranes in studying their structure and function.

To engineer future devices, control them with light and create new properties through systematic modifications, we first need to understand the structural transformations of monolayers on the atomic level," said Stanford researcher Ehren Mannebach, the study's lead author.

But monolayers can only reduce evaporation loss by 10--40 per cent at best compared with shade or plastic covers, which can reduce losses by 70-95 per cent according to a study by the National Centre for Engineering in Agriculture at the University of Southern Queensland, Toowoomba.

By specifying a tough three-layer anticorrosion coating system instead of a monolayer epoxy coating, the engineers made a decision that minimized costs and assured maximum reliability of corrosion protection.

We will devise and build a novel resonant inelastic x-ray scattering (RIXS) instrument capable of determining the dispersion relations of electronic collective modes in 4d-metal compounds with full momentum-space coverage, high energy resolution, and monolayer sensitivity.

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